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Add documentation for calculated columns #162

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Nov 15, 2023
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Add documentation for calculated columns #162

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11 changes: 11 additions & 0 deletions README.rst
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Expand Up @@ -208,6 +208,17 @@ This can be e.g. done by using ``alembic``:
op.get_bind().execute("RENAME COLUMN TAB.valid_tmp to valid")
# optionally, remove also the server default by using alter column

Computed columns
~~~~~~~~~~~~~~~~
SAP HANA supports two computed/calculated columns:

* <col> AS <expr>: the column is fully virtual and the expression is evaluated with each SELECT
* <col> GENERATED ALWAYS AS <expr>: the expression is evaluated during insertion and the value
is stored in the table

By default, sqlalchemy-hana creates a ``GENERATED ALWAYS AS`` if a ``Computed`` column is used.
If ``Computed(persisted=False)`` is used, a fully virtual column using ``AS`` is created.

Alembic
-------
The sqlalchemy-hana dialect also contains a dialect for ``alembic``.
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